Remote sparse distributed sensors to image bondline defects between a composite panel and a stiffener

Abstract

Adhesive bonding is widely used to fix skins to reinforcing elements, like stiffeners, in aerospace composite structures. A well-cured bond offers uniform stresses, good joint strength, and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. In the previous work by the authors, ultrasonic feature guided wave (FGW) has been discussed as a screening tool for quick inspection of the bondline between a CFRP panel and stiffener. However, it was found that material damping and weak reflections by defects along the feature makes the use of a single transducer in pulse echo mode quite inefficient. In this study, a structural health monitoring (SHM) technique is proposed to inspect the bondline, based on the radiation of plate modes into the CFRP panel when an incident FGW interacts with a defect in that bondline. A sparse array of sensors organized on the CFRP panel away from the stiffener was used to capture the waves radiating into the panel. A synthetic focusing method was applied to process the recorded signals for imaging the damaged area in the bond. Adhesive defects with varying dimensions were successfully identified in 3D FE simulations, with supporting experimental results to validate this method.